chore(web): remove Bootstrap, add 4 replacement utility rules, delete dead style.css

Bootstrap CSS (232 KB) and Bootstrap JS (39 KB) were loaded for only
4 utility classes (.btn-sm, .btn-dark, .btn-warning, .shadow).
Replace with 4 targeted rules in myai.css and delete both files.

style.css (784 lines) was unreferenced by any HTML page — delete it.

Closes #31

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/wwwroot/css/myai.css

@@ -218,6 +218,12 @@ img {
     color: #fff
 }
 
+/* Bootstrap replacement utilities (btn-sm, btn-dark, btn-warning, shadow) */
+.btn-sm { padding: 5px 10px; font-size: .875rem }
+.btn-dark { background: #1e2730; color: #fff; border-color: #1e2730 }
+.btn-warning { background: #ffc107; color: #212529; border: 0 }
+.shadow { box-shadow: 0 .5rem 1rem rgba(0,0,0,.15) !important }
+
 .section {
     padding: 76px 0
 }